1. Field of the Invention
The present invention relates generally to a serigraphic machine and more particularly to apparatus for positioning and holding a screen carrier in a serigraphic machine adapted primarily for the manufacture of integrated circuits and substrates therefor.
2. Description of the Prior Art
To obtain circuits which are more and more efficient and to assemble them in a reduced volume, that is to say, to make them more compact, manufacturers have been led to utilize microplates for integrated circuits commonly known as chips and to mount them on the substrates in which the conductors are distributed in several separate planes separated one from the other by insulating layers, except for certain predetermined points where conductive columns assure electric connection between the conductors situated in the different planes.
The manufacture of each substrate is generally done with a serigraphic machine which provides for depositing on an electrically insulating support, which may be, for example, alumina, a series of alternating conductive and insulating layers, each layer being obtained by application of, through a screen of which the transparent portion corresponds to that of the layer to be obtained, the pasty material which is either conductive or insulating. After each application of a layer, this support is withdrawn from the serigraphic machine and placed in a furnace maintained at a high termperature. This heating step permits the layer to solidify and form a layer having a metallic or insulating character which adheres strongly to the base support or to the preceding layer.
In order to obtain proper electric connections between the conductors situated in the different planes, it is indispensable not only to have the screens (which are mounted successively in the serigraphic machine for deposit of the layers) positioned with very great precision one with respect to the others, but to equally insure that each time the support is put in place in the serigraphic machine to receive a layer, it is positioned with precision with respect to the screen.
Ihe screen is generally carried by a support which is positioned and maintained in position in the serigraphic machine by a positioning and holding device. Such a device is described in French Application No. 80.27417 filed in France on Dec. 24, 1980 in the name of Compagnie Internationale Pour L'Informatique Cii-Honeywell Bull, assignee of the present invention, and corresponding application for patent in the United States, Ser. No. 331,283, filed concurrently herewith. The subject matter of said application is hereby incorporated by reference. The device described in this application comprises two parts respectively carried by the screen carrier and by the serigraphic machine. At the level of the screen carrier the device comprises two bores, and at the level of the serigraphic machine it comprises two brooches or reference pins with vertical axes adapted to engage in the two bores of the screen carrier to assure the positioning of the screen carrier while the holding in position of the screen carrier is carried out by simple blocking screws.
However, it is important to note that the mounting of the screen carrier in the serigraphic machine is carried out by displacement of the screen carrier in a direction perpendicular to its plane to bring it from a lower position to an upper position where the fixed reference pins, with vertical axis, enter in the corresponding bores of the screen carrier. In such a serigraphic machine, the screen support is placed on a movable table movable along a vertical axis and displacement of the table is controlled by a jack. The screen carrier is put in place by immovable vertical columns which provide only approximate centering of the screen support.
The final positioning of the screen support in the serigraphic machine is obtained by the precision of manufacture of the reference pins and of the bores, machining carried out to about several microns. In other words, the precision of positioning of the screen carrier can be obtained only under the condition that it has minimum play between each reference pin and the corresponding bore of the screen carrier. Because of the imprecision of the mounting of the screen carrier, the repeated cooperation between the reference pins and the bores of the screen carrier causes a progressive and inevitable wear of these parts. As a result of this wear, the necessary precision of positioning of the screen carrier in the serigraphic machine is eventually lost. Finally, it is to be noted that the mounting of the screen carrier requires the use of mounting columns which encumbers the structure of the serigraphic machine.